1. Field of the Invention
The present invention relates to a shock absorbing structure of a propeller shaft used for a vehicle or the like.
2. Description of the Related Art
In a conventional shock absorbing structure of a propeller shaft, there is a structure in which an outer race of a tripod universal joint is formed in a rear end of a first propeller shaft, an inner shaft portion is formed in a front portion of a coupling member, tripods corresponding to three bearings protruding in a radial direction is spline fitted to a leading end portion of the inner shaft portion, and the respective tripods are slidably fitted to three grooves formed on a cylindrical inner peripheral surface of the outer race while being directed in an axial direction so as to structure the tripod universal joint, thereby executing a power transmission at a constant velocity (refer to Japanese Patent Application Laid-open No. 10-250390 (patent document 1)). A protruding portion for positioning an inner ring portion of the tripod is formed in the inner shaft portion, the tripod pressed by the outer race comes into collision with the protruding portion so as to break the protruding portion in a predetermined case, and the tripod slides on the inner shaft portion and the coupling member, thereby absorbing a shock.
In this case, in the shock absorbing structure of the propeller shaft mentioned above, a thickness of the protruding portion is small, and in the case that an induction hardening is applied, there is a risk that the protruding portion gets chipped or is broken at a time of carrying the coupling member or assembling the propeller shaft. Further, there is a problem that a crack is generated in the protruding portion at a time of hardening. Further, when the tripod pressed by the outer race is brought into contact with the protruding portion and comes into collision with the protruding portion, the tripod and the protruding portion are brought into contact with each other and the protruding portion is broken. Accordingly, there is a problem that a dispersion is generated in a breaking load of the protruding portion due to a dimensional error of the thickness of the protruding portion, a dimensional error of the tripod, an error of a fitting frictional force of the tripod with respect to the inner shaft, or the like.